Compact short-wavelength light sources are required in order to achieve high density for optical disks and high resolution for displays. Coherent light sources using a semiconductor laser and a quasi phase matching (referred to as “QPM” in the following) optical waveguide-type second harmonic generation (referred to as “SHG” in the following) device (optical waveguide-type QPM-SHG device) have received attention as compact short-wavelength light sources (see Yamamoto et al., Optics Letters, Vol. 16, No. 15, 1156 (1991)).
FIG. 20 shows a diagram showing the configuration of an SHG blue light source using an optical waveguide-type QPM-SHG device. As shown in FIG. 20, a wavelength-variable distributed Bragg reflector (referred to as “DBR” in the following) semiconductor laser 54 having a DBR region is used as the semiconductor laser. The wavelength-variable DBR semiconductor laser 54 is a 0.85 μm band 100 mW level AlGaAs wavelength-variable DBR semiconductor laser, and includes an active layer region 56, a phase adjustment region 57 and a DBR region 58. The oscillation wavelength can be changed continuously by controlling the currents injected into the phase adjustment region 57 and the DBR region 58 at a constant ratio.
The optical waveguide-type QPM-SHG device 55 serving as the second harmonic generation device is made of an optical waveguide 60 and periodic polarity inversion regions 61 formed on an X-cut MgO-doped LiNbO3 substrate 59. The optical waveguide 60 is formed by proton exchange in pyrophosphoric acid. Moreover, the periodic polarity inversion regions 61 are fabricated by forming a comb-shaped electrode on the X-cut MgO-doped LiNbO3 substrate 59 and applying an electric field.
In the SHG blue light source shown in FIG. 20, 75 mW of laser light are coupled into the optical waveguide 60 for 100 mW of the laser output. By controlling the amounts of current injected into the phase adjustment region 57 and the DBR region 58 of the wavelength-variable DBR semiconductor laser 54, the oscillation wavelength is fixed within the phase matching wavelength tolerance width of the optical waveguide-type QPM-SHG device 55. The use of this SHG blue light source provides about 25 mW of blue light of 425 nm wavelength, and the obtained blue light, when lateral mode is the TE00 mode, has focusing property of the diffraction limit and also low noise performance with a relative noise field intensity of −140 dB/Hz or less, which are characteristics suitable for reproducing optical disks.
When the optical waveguide-type QPM-SHG device serving as the second harmonic generation device is evaluated for the output characteristics of blue light with respect to the wavelength of the fundamental wave, it can be seen that its wavelength width at which the output of blue light is half (wavelength tolerance width for phase matching) is as small as about 0.1 nm. This presents a significant problem in obtaining a stable output of blue light. In order to solve this problem, conventionally, the wavelength-variable DBR semiconductor laser is used as the fundamental wave and the wavelength (oscillation wavelength) of the fundamental wave is fixed within the phase matching wavelength tolerance width of the optical waveguide-type QPM-SHG device, thereby realizing a stable output of blue light.
Ordinarily, the oscillation wavelength of the semiconductor laser source changes with the ambient temperature, and the optimum wavelength of the optical waveguide-type QPM-SHG device also changes with the ambient temperature. Therefore, conventionally, the temperatures of the semiconductor laser source and the optical waveguide-type QPM-SHG device are maintained constant by using a Peltier device or the like, thereby stabilizing the output of blue light.
However, when considering the installation in optical information processing equipment, such as optical disk devices and laser printers, the average output power changes every instant during operation. In this case, even when the ambient temperature is maintained constant by using a Peltier device or the like, the amount of heat generated by the semiconductor laser source changes, so that the temperature of the semiconductor laser source itself changes and hence the oscillation wavelength changes, making it impossible to obtain a stable output of blue light.
Moreover, in the case of omitting the use of a temperature control device such as a Peltier device for the purpose of downsizing the device, the fluctuation in the ambient temperature further increases, leading to a change in the output of the optical waveguide-type QPM-SHG device.